This project investigates the roles of genetic mechanisms and individual experience in the development of normal vocal behavior in nonhuman primates and human infants. During the past year new findings were obtained from 4 sets of studies, which are summarized below. 1. Development of vocal reptertoire in marmosets: significant contextual differences in the production rates of two distinctive vocal patterns (scream and tsik calls) were found in infant marmosets. 2. Infant retrieval in common marmosets: an important role for recent experience was found in studies of adult/older sibling retrieval of marmoset neonates. Adults and subadults without neonates present in their family group failed to retrieve 1-4 week-old infants, whereas adults and subadults with infants present in their family groups retrieved infants with latencies no greater than 180 seconds. 3. Acoustic differentiation of recruitment screams in rhesus macaques: a group of 35 rhesus macaques who had no previous contact with feral individuals produced agonistic scream vocalizations with the same acoustic subtypes and in the same contexts as feral rhesus macaques living on Cayo Santiago Island off Puerto Rico. This suggests that production and context-dependent usage of this subclass of vocalizations is under genetic control. 4. Development of affect during social separation in infant rhesus macaques: a positive correlation between heart rate and scream rate and a negative correlation between heart rate and coo rate was found during 30 min. social separations at 3, 4, and 5 months of age. 5. Maturation of social behavior in common marmosets: longitudinal observations of juveniles in their natal family groups and following pairing with an opposite=sex peer revealed significant context differences in vocal play, and sexual behavior, and few sex differences in behavior in either context.